1. Field of the Invention
The invention relates to a sensor strip, and more particularly to an electrochemical sensor strip.
2. Description of the Related Art
Electrochemical sensor strips have been widely used in analysis of various kinds of fluids. Generally, such sensor strips include a container for receiving fluid to be tested, a reaction reagent layer fixed in the container for reacting electrochemically with a target analyte in the fluid, and an electrode system mounted in the container. A reaction between the reagent and the analyte will generate an output signal involving an electric characteristic parameter, and the concentration of the analyte in the fluid can be determined based on the electric characteristic parameter. The electrode system includes a counter electrode, a working electrode, a reference electrode and a detecting electrode. By varying the reaction reagent used in the container, different kinds of electrochemical sensors may be fabricated for detection of various analytes, such as, blood sugar, urine acidity, cholesterol concentration in blood, and heavy metals, pesticides, and other toxic components in polluted water.
To ensure accurate detection results of an electrochemical sensor, interfering substances that may interfere with the detection result are usually removed from the fluid to be analyzed beforehand. Taking whole blood as an example, since red blood cells can affect the detected result of the sensor, a membrane filter is disposed between an opening of the container and the reaction reagent layer to filter and separate the red cell bloods from blood plasmas in the whole blood. U.S. Pat. No. 5,139,685 discloses a glass fiber membrane filter for separating blood cells and blood plasmas without the need of centrifugal separation. However, when the amount of the blood under test is small, the amount of the filtrate produced by the filter can be considerably small, and a certain pressure has to be applied to completely filter out the blood cells in the whole blood. Furthermore, homolysis is likely to occur when subjected to pressure, thereby resulting in an inaccurate result. Moreover, the blood plasmas absorbed by the filter tends to slow down the flow rate of the blood plasmas, and hence a longer time is needed to obtain the sensing result.
Referring to FIG. 1, an electrochemical sensor 1 disclosed in U.S. Pat. No. 6,319,719 is provided with a plurality of spaced apart crescent-shaped first, second and third obstructions 11, 12 and 13 disposed in a fluid passage for retarding and removing red blood cells from a whole blood and for permitting blood plasma to quickly flaw through the obstructions 11, 12, 13 and to move to a reaction reagent layer 14. However, this sort of design requires a larger space for providing the obstructions 11, 12, 13 in an amount sufficient for separation resulting in the need of a relatively large amount of blood to obtain a detection result smoothly. Moreover, because an etching process is needed to provide the obstructions 11, 12, 13 with particular size, shape and spacing, fabrication of such a sensor requires a relatively long production time and a relatively high production cost.
U.S. Pat. No. 6,966,977 discloses an electrochemical sensor strip that includes a filter to filter a fluid to be analyzed and to guide blood plasmas to move to a reaction reagent layer and an electrode system. The filter has, at an upstream side thereof, a cross-sectional area larger than a cross-sectional area of an opening of a sample solution supply pathway, and also larger than a cross-sectional area at a downstream side thereof. The sample solution supply pathway has a cross-sectional area equal to or smaller than the cross-sectional area of the opening of the sample solution supply pathway. However, the electrochemical sensor strip has the following disadvantages:
1. The downstream side of the filter cannot extend through the open end of the sample solution supply pathway, otherwise the filter will contact the electrode system and cause an adverse affect to detection results. Moreover, although variation of the cross section of the filter enables the blood plasmas to quickly reach a front end of the filter, a relatively long time is needed to have the fluid accumulated to an amount sufficient for detection.
2. Since the sample solution supply pathway is sized to match with the electrode system and the reaction reagent layer, the size thereof cannot be varied easily. In order for the cross section of the upstream side to be larger than that of the downstream side, the filter must have a wide or thick size. An increase in size of the filter results in the need of an increased fluid amount and an increased sensing time.
U.S. Pat. No. 5,628,890 discloses an electrochemical sensor strip in which filter layers and electrodes are arranged in a stack, and all electrodes (three electrodes) are covered by the filter layers. The structure disclosed therein can reduce the distance between the electrode unit and the point where the fluid is dropped and thereby increase the speed of obtaining the sensing result. However, the number of the filter layers has to be increased, and an electric isolation layer made of a hydrophobic material is additionally needed. The resulting multi-layered structure has to be made up of a number of component parts and requires a micropump or a pressurizing force for driving the fluid.